It is known to develop a film by mounting it in a holder within a closable cassete provided with at least one hole having a so-called light gate that permits a fluid to enter or leave the cassette without letting light pass. The holder carrying the film is secured in this cassette in a darkroom and thereafter the closed cassette may be immersed in the various film-treatment baths in an illuminated area. Such an arrangement is frequently used by amateurs or small-capacity photo labs.
Usually the cassette or container in which the film is held is forcibly immersed by the operator in the treatment bath and held there until it slowly fills up with the treatment liquid. Thereafter the cassette is agitated to bring the liquid into contact with the film, and at the end of the treatment time the entire cassette is slowly raised out of the bath. The immersion and extraction time is usually relatively long because the light gate prevents rapid fluid passage through the holes into the cassette.
Since on to three liters of fluid are generally needed for the treatment, the time necessary for this fluid to flow into or out of the cassette is long. In addition it is necessary that the fluid be circulated to some extent within the cassette, usually by means of a paddle-type mixing device within the cassette or simply by jiggling or vibrating the cassette manually, in order to achieve even results. Even with the best of such arrangements, however, the film develops irregularly in many instances since it is impossible to obtain a uniform treatment of the entire exposed film surface.
The extended time it takes the cassette to fill and empty also makes it difficult exactly to control how long the film is exposed to the particular treatment liquid. The film or film portion lowest in the cassette is invariably treated substantially longer than the rest of the film, as the bottom of the cassette is the first part to fill up and the last part to empty.